Each party was offering one plan it said would keep terrorists from obtaining firearms and a second bolstering the existing system of background checks for gun purchases. Democrats said the GOP proposals were unacceptably weak, Republicans faulted the Democrats' plans as overly restrictive and all four faced likely defeat in largely party-line votes.

That Monday's four roll-call votes were occurring at all was testament to the powerful political currents buffeting lawmakers after gunman Omar Mateen's June 12 attack on a gay nightclub. The 49 victims who died made it the largest mass shooting in recent U.S. history, topping the string of such incidents that have punctuated recent years.

In addition, the FBI said Matteen — a focus of two terror investigations that were dropped — described himself as an Islamic soldier in a 911 call during the shootings. That let gun control advocates add national security and the specter of terrorism to their arguments for firearms curbs.

Even so, the expected rejection of the proposals underscored the pressure on each party to give little ground on the emotional gun issue going into November's presidential and congressional elections. It also highlighted the potency of the National Rifle Association, which was urging its huge and fiercely loyal membership to lobby senators to oppose the Democratic bills.

"If they learn they can politicize tragedy and win, they'll be back blaming law-abiding gun owners each time a criminal or terrorist attacks the innocent," the NRA said in an email to supporters.

Gun control groups were also working Capitol Hill, with relatives of victims of past mass shootings and others visiting lawmakers and planning to watch the day's debate from the Senate visitors' gallery.

Former Rep. Gabrielle Giffords, D-Ariz., gravely wounded in a 2011 mass shooting, and husband Mark Kelly said Monday's votes would show voters "whether their elected leaders in the Senate are on the side of the corporate gun lobby or the vast majority of Americans," whom polls show favor some gun curbs.

Under special pressure were GOP senators facing re-election this fall from swing states like Pennsylvania, New Hampshire and elsewhere.

Monday's votes were coming after Sen. Chris Murphy, D-Conn., led a near 15-hour filibuster last week demanding a Senate response to the Orlando killings. Murphy entered the Senate shortly after the December 2012 massacre of 20 first-graders and six educators in Newtown, Connecticut, but that slaughter and others in San Bernardino, California, and Charleston, South Carolina, have failed to spur Congress to approve significant gun curbs. The last were enacted in 2007, when the background check system was strengthened after that year's mass shooting at Virginia Tech.

Because of Mateen's self-professed loyalty to extremist groups and his 10-month inclusion on a federal terrorism watch list, proposals aimed at blocking terrorists from getting guns were in the spotlight. One proposal by Sen. Dianne Feinstein, D-Calif., would let the government block many gun sales to known or suspected terrorists.

People buying firearms from federally licensed gun dealers can currently be denied for several reasons, chiefly for serious crimes or mental problems. There is no specific prohibition for those on the terrorist watch list, which the FBI said in 2014 had 800,000 names on it, and no background checks are required for anyone buying guns privately online or at a gun show.

The GOP response to Feinstein was an NRA-backed plan by Sen. John Cornyn, R-Texas. It would let the government deny a sale to a known or suspected terrorist — but only if prosecutors could convince a judge within three days that the would-be buyer was involved in terrorism.

The Feinstein and Cornyn amendments would both require notification of law enforcement officials if people like Mateen who had been a subject of a terrorism investigation within the past five years were seeking to buy firearms.

Republicans said Feinstein's proposal gave the government too much unfettered power to deny people's constitutional right to own a gun. They also noted that the terrorist watch list has historically mistakenly included people. Democrats said the three-day window that Cornyn's measure gave prosecutors to prove their case made his plan ineffective.

The Senate rejected similar plans Feinstein and Cornyn proposed last December, a day after the attack in San Bernardino killed 14 people.

Murphy had a proposal widely expanding the requirement for background checks, even to many private gun transactions, that would leave few loopholes.

Senate Judiciary Committee Chairman Charles Grassley, R-Iowa, would increase money for the background check system. Like Murphy's measure, it would also prod states to do a better job of sending records to the FBI, which operates the background check system, of felons and others barred from buying guns.

Grassley's proposal would also clarify language that prohibits some people with mental health issues from buying a gun. Democrats claimed that language would actually roll back some current protections.

Separately, moderate Sen. Susan Collins, R-Maine, was laboring to fashion a bipartisan bill that would prevent people on the no-fly list — with just 64,000 names in 2014 — from getting guns. There were no signs Monday that it was getting wide support or would receive a vote.